Some systems ensure surveillance of large air/sea/ground areas in the purpose of detecting objects such as aircrafts, vessels or vehicles, whether they are moving or not. They may basically consist of a network of radar systems. While the existing techniques focus on accurately determining range, i.e. the distance of the detected objects relatively to the radar, it is a remaining issue to better estimate the distance between the detected objects, so-called “cross-range”. Indeed, the existing surveillance systems provide quite a low cross-range resolution.
Synthetic Aperture Radar (SAR) techniques have been used in the past in an attempt to estimate the cross-range between static objects for imaging of large ground areas by use of moving radars. In these techniques, the so-called “swath” is the area in which the cross-range between two objects can be estimated.
Klausing and Keydel disclosed a rotating SAR (ROSAR) mounted on the rotating wings of a helicopter for imaging the ground (“Feasibility of SAR with rotating antennas”, 1990, IEEE International Radar Conference, pp. 51-56). In Klausing and Keydel, the swath is a ring-shaped area illuminated by the radar beam if the helicopter flies a stationary position. Klausing and Keydel also disclose an algorithm adapted to the ring-shaped swath. A major drawback is that it is difficult to fly a helicopter at a true stationary location.
Soumekh disclosed a circular SAR (CSAR) mounted under an airplane flying a circle for imaging the ground (“Reconnaissance with Slant Plane Circular SAR Imaging”, 1996, IEEE Transactions on Image Processing, V5/no8, pp. 1252-1265). In Soumekh, the swath is a disc-shaped area illuminated by the radar beam if the airplane always flies the same circle. Soumekh also discloses an algorithm adapted to the disc-shaped swath. A major drawback is that it is difficult to fly a true circle at a constant altitude and speed.
Unfortunately, neither of these existing solutions is adapted to ensure surveillance of a large area, nor adapted to provide a permanent surveillance, nor adapted to accurately estimate cross-range between objects separated in the vertical dimension, such as aircrafts.